1. Field of the Invention
The present invention relates to a method of manufacturing a piezoelectric resonator. More particularly, the present invention relates to a method of manufacturing a piezoelectric resonator which is provided in electronic components, such as an oscillator, a discriminator, and a filter, and which uses mechanical resonance of a piezoelectric body.
2. Description of the Related Art
FIG. 26 is a perspective view showing an example of a conventional piezoelectric resonator. A piezoelectric resonator 1 shown in FIG. 26 includes a piezoelectric substrate 2, for example, having a rectangular plate shape. The piezoelectric substrate 2 is polarized along the thickness direction thereof. Electrodes 3 are provided on both major surfaces of the piezoelectric substrate 2. As a result of inputting a signal between these electrodes 3, an electric field is applied along the thickness direction of the piezoelectric substrate 2, causing the piezoelectric substrate 2 to vibrate along the length direction thereof.
The piezoelectric resonator shown in FIG. 26 is an unstiffened type, in which the vibration direction is different from the electric-field direction and the polarization direction. An electro-mechanical coupling coefficient of a piezoelectric resonator of such an unstiffened resonator is lower than that of a stiffened piezoelectric resonator, in which the electric-field direction, the polarization direction, and the vibration direction coincide with each other. Therefore, in the unstiffened type piezoelectric resonator, the difference .DELTA.F between the resonance frequency and the anti-resonance frequency is relatively small. This causes a very small bandwidth when such an unstiffened piezoelectric resonator is used for a filter. Therefore, the degree of characteristic design freedom is small in such an unstiffened piezoelectric resonator and electronic components incorporating such a resonator.
Furthermore, in the piezoelectric resonator shown in FIG. 26, a primary resonance of a length mode is used. However, due to the structure of the resonator shown in FIG. 26, an odd-number multiple high-order mode, such as a third order mode or a fifth order mode, and a large spurious vibration of a width mode, are generated.
Japanese Patent Application No. 8-110475, filed by the applicant of the present invention, describes a piezoelectric resonator having a multilayered base structure having a longitudinal direction which is provided as a result of a plurality of piezoelectric layers and a plurality of electrodes being alternately stacked and laminated. The plurality of piezoelectric layers are polarized along the length direction of the base, and a fundamental vibration of a longitudinal vibration is excited. The piezoelectric resonator of such a multilayered structure is a stiffened type resonator, in which the polarization direction, the electric-field direction, and the vibration direction are the same. As a result, such a stiffened resonator has spurious emissions that are smaller than that of an unstifffened type resonator, and the difference .DELTA.F between the resonance frequency and the anti-resonance frequency is large in this stiffened type resonator.
Next, an example of a piezoelectric resonator having such a multilayered structure will be described in detail. FIG. 1 is a perspective view showing an example of a conventional piezoelectric resonator having a multilayered structure, to provide a background against which-the present invention will be compared later. FIG. 2 is a schematic view of the piezoelectric resonator. FIG. 3 is a plan view of the essential portion of the piezoelectric resonator.
A piezoelectric resonator 10 in FIG. 1 having such a multilayered structure includes a base 12, for example, having a rectangular body. The base 12 includes a plurality of piezoelectric layers 12a, which are formed from, for example, a piezoelectric ceramic, and are multilayered. In the plurality of piezoelectric layers 12a in the intermediate portion along the length direction of the base 12, a plurality of internal electrodes 14 are disposed on each of the two main surfaces so as to be perpendicular relative to the length direction of the base 12. Therefore, a plurality of internal electrodes 14 are disposed and spaced apart in a direction that is perpendicular to the length direction of the base 12 and along the length direction of the base 12. Also, the plurality of piezoelectric layers 12a in the intermediate portion along the length direction of the base 12, as indicated by the arrows in FIG. 2, are polarized along the length direction of the base 12 so that adjacent piezoelectric layers are oppositely polarized relative to each other on both sides of the respective internal electrodes 14. However, the piezoelectric layers 12a of both end portions along the length direction of the base 12 are not polarized. In this base 12, the internal electrodes 14 are exposed at four side surfaces which are parallel to the length direction of the base 12.
A groove 15 which extends along the length direction of the base 12 is formed on one side surface of the base 12. The groove 15 is formed in the center in the width direction of the base 12, dividing one side surface of the base 12 into two portions. Furthermore, as shown in FIG. 2, a first insulation film 16 and a second insulation film 18 are disposed on the side surfaces divided by the groove 15. On one side divided by the groove 15 on the side surface of the base 12, every alternate exposed portion of the internal electrodes 14 is covered by the first insulation film 16. Also, on the other side divided by the groove 15 on the side surface of the base 12, the exposed portions of the internal electrodes 14 that are not covered by the first insulation film 16 on one side of the groove 15 are covered by the second insulation film 18.
Furthermore, at the portions where the first and second insulation films 16 and 18 of the base 12 are disposed, that is, on both sides of the groove 15, two external electrodes 20 and 22 are disposed. Therefore, the internal electrodes 14 that are not covered by the first insulation film 16 are connected to the external electrode 20, and the internal electrodes 14 that are not covered by the second insulation film 18 are connected to the external electrode 22. That is, adjacent internal electrodes 14 are connected to the external electrode 20 and the external electrode 22, respectively.
In this piezoelectric resonator 10, the external electrodes 20 and 22 are used as input and output electrodes. In the intermediate portion along the length direction of the base 12, since the section between adjacent internal electrodes 14 is polarized and an electric field is applied between the adjacent internal electrodes 14, the section is piezoelectrically active. Since mutually opposite voltages are applied to the portions of the base 12 which are mutually oppositely polarized, the base 12 expands or contracts in the same direction as a whole. Therefore, in the entire piezoelectric resonator 10, a fundamental vibration in a longitudinal vibration mode, in which the center portion along the length direction of the base 12 is a node, is excited. Both end portions along the length direction of the base 12 are not polarized, and an electric field is not applied thereto because no electrode is disposed at the end portions. Therefore, both end portions are piezoelectrically inactive.
In this piezoelectric resonator 10, the polarization direction of the base 12, the electric-field direction applied by the input signal, and the vibration direction of the base 12 are the same. That is, this piezoelectric resonator 10 is a stiffened piezoelectric resonator. This piezoelectric resonator 10 has an electro-mechanical coupling coefficient greater than that of an unstiffened type, such that the polarization direction, the electric-field direction, and the vibration direction are different from each other. Therefore, in this piezoelectric resonator 10, it is possible to increase the selectable width of the difference .DELTA.F between the resonance frequency and the anti-resonance frequency in comparison with an unstiffened piezoelectric resonator. Therefore, in this piezoelectric resonator 10, it is possible to obtain a characteristic with a larger bandwidth than that of an unstiffened resonator. Furthermore, this piezoelectric resonator 10 has spurious emissions which are smaller than that of an unstiffened resonator. In addition, in this. piezoelectric resonator 10, since the external electrodes 20 and 22 are disposed on a single common side surface thereof, the resonator 10 can be surface-mounted onto, for example, an insulator substrate.
A method of manufacturing this piezoelectric resonator 10 will be described below with reference to FIGS. 4 to 13. In these figures, for convenience of description, the number of layers of green sheets which form the piezoelectric layers 12a does not coincide with the number of layers of the piezoelectric layers 12a which form the piezoelectric resonator 10 shown in FIGS. 2 and 3. However, the following manufacturing process is the same regardless of the number of piezoelectric layers.
When manufacturing this piezoelectric resonator 10, as shown in FIG. 4, a green sheet 30 is prepared first. Conductive paste containing, for example, silver, palladium, an organic binder, and the like, is coated onto one surface of the green sheet 30, forming a conductive paste layer 32. The conductive paste layer 32 is formed on the entire surface excluding one end side of the green sheet 30. A plurality of the green sheets 30 are stacked in layers. At this time, the green sheets 30 are multilayered so that alternate end portions which are not formed with the conductive paste layer 32 are disposed in mutually opposite directions. Furthermore, since a conductive paste is coated onto the opposing side surfaces of the multilayered body and then sintered, a multilayered base 34 such as that shown in FIG. 5 is formed.
Inside the multilayered base 34, as a result of the conductive paste layer 32 being sintered, a plurality of internal electrodes 36 are formed. These internal electrodes 36 are alternately exposed at the opposing portions of the multilayered base 34. Then, in the opposing portions of the multilayered base 34, electrodes 38 and 40 for polarization are formed, to which each alternate internal electrode 36 is connected. By applying a direct-current voltage to these polarization electrodes 38 and 40, a polarization process is performed on the multilayered base 34. At this time, inside the multilayered base 341 a direct-current high electric-field is applied between adjacent internal electrodes 36, and the directions of the applied electric field are opposite to each other. Therefore, the multilayered base 34 is polarized in mutually opposite directions on both sides of the internal electrodes 36, as indicated by the arrows in FIG. 5.
Next, as indicated by the dotted line in FIG. 6, the multilayered base 34 is cut by a dicer or the like in such a manner as to intersect at right angles to the plurality of internal electrodes 36 and the polarization electrodes 38 and 40. As a result, a multilayered body 42 such as that shown in FIG. 7 is formed.
Then, as shown in FIG. 8, an insulation film 44 is arranged in such a manner as to form a checkered pattern on one main surface of the multilayered body 42. In this case, in one row in the vertical direction with respect to the internal electrodes 36 of a checkered pattern, the insulation film 44 is disposed on alternate internal electrodes 36 in the vertical direction with respect to the internal electrodes 36 of the multilayered body 42. Also, in a row which is vertical with respect to the adjacent internal electrodes 36 of the multilayered body 42, the insulation film 44 is formed on the internal electrodes 36 which are not covered with the insulation film 44 in the adjacent row.
Thereafter, in this multilayered body 42, on the entire surface where the insulation film 44 is formed, as shown in FIG. 9, an external electrode 48 is formed by sputtering or the like.
Next, in the multilayered body 42, the groove 15 is formed so as to intersect at right angles to the surface of the internal electrodes 36 by a dicing machine in the portion indicated by the one-dot-chain line in FIG. 10, specifically, in the portion between the one-dot-chain lines of FIG. 11, that is, on the main surface of the multilayered body 42 in the boundary portion of adjacent rows of the insulation film 44 arranged in a checkered pattern, and further, by cutting the multilayered body 42 as shown in FIG. 12, the piezoelectric resonator 10 shown in FIGS. 1 and 2 is formed in the portion indicated by the dotted line in FIG. 10, specifically, in the portion between the one-dot-chain lines of FIG. 11, that is, in the intermediate portion of these grooves 15.
However, in the above-described method, when the position at which the groove 15 is formed in the multilayered body 42 is deviated by 1/2 or more of the edge thickness (corresponding to the width of the groove 15) of a dicing machine, for example, as shown in FIG. 13, the groove 15 is deviated from the boundary of the adjacent rows of the insulation film 44. In this case, in the piezoelectric resonator 10 to be formed, as shown in FIG. 14, the internal electrodes 36 (14) to be insulated are not insulated completely by the insulation film 44 (16), and the section between external electrodes 48 (20) and 48 (22) is short-circuited. In this manner, in the above-described method, the groove 15 must be formed so as to include the edge of the insulation film 44, and the position at which the groove 15 is formed requires high accuracy, making it difficult to manufacture the piezoelectric resonator 10 with a high yield of non-defective products.